Callback application
--------------------

This application FRs the caller with 468 Busy, and calls back 
after some time. If the callback call is established, the called 
party may enter a number, to which the call is connected in B2BUA 
mode with media relay.

Both the callback call to the caller, and the outgoing call 
to the callee) are made and authenticated using a configurable
account (From, domain and authentication).

Allowed callback users may be specified with a regexp in the
configuration.

Used together with reg-agent this app can greatly reduce your
mobile phone bill...